In many communication systems, the received signal amplitude varies widely. For example, in magnetic recording channels, replay signal levels may vary by more than 20 dB due to the combined properties of the heads, media and preamplifiers. In such cases, automatic gain control (AGC) is ubiquitously employed to limit the signal variation. Signal levels after AGC become more consistent with a much smaller dynamic range, which facilitates further signal processing.
There are primarily two different approaches for automatic gain control; either non-data-aided (NDA) or data-aided (DA), depending on whether decisions on the transmitted symbols are exploited. Typically, data-aided AGCs are more efficient than non-data-aided AGCs, and hence data-aided AGCs are more often employed in modem communication systems.
While many types of AGCs are known, there remains a need for an automatic gain control that can take advantage of the particular characteristics of data storage systems to provide improved performance.